In the prior art, it has been required to connect two display device, such as CRT (Cathode Ray Tube) display device and LCD (Liquid Crystal Display) device to a personal computer. Such a configuration is shown in FIG. 2, wherein an output of a personal computer 20 is connected to the CRT display device 21 and the LCD device 25 through a digital-analog converter 22 and a LCD controller 23, respectively. Gray levels displayed by the two display devices differ from each other. For example, the CRT display device 21 displays 64 gray levels and the LCD device 25 displays 16 gray levels. The personal computer 20 supplies 64 bit signal for each picture element of the display devices on the output line. The 6 bit signal represents the 64 gray levels of the CRT display device 25. It was required to display the 64 gray levels on the LCD device 25, each pel of which displays only the 16 gray levels. To this end, LCD controller 23 responds each 6 bit signal to generate four 4 bit signals. The 4 bit signal represents the 16 gray levels displayed by one pel of the LCD device 25. And, four pels of the LCD device 25 are used to display the four 4 bit signals, so that if a size of one image supplied from the personal computer 20 is 320.times.200 pels (dots), the CRT display device 21 has 320.times.200 dots and the LCD device 25 has 640.times.400 dots, as shown in FIG. 3. The controller described above is disclosed in Japanese patent application 63-236291 corresponding to U.S. Pat. No. 5,059,962, assigned to the same assignee as that of the present application. Blocks assigned with reference number 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 12, 21 and 25 in FIG. 1 show the controller of the Japanese patent application 63-236291. Detail operation for generating the four 4 bit signals for the LCD device 25 based upon one 6 bit signal for one pel of the CRT display device 21 will be described hereinafter since the present invention also uses the algorithm of the Japanese patent application 63-236291.